<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>debugger</title>
    <script src="./js/x.js"></script>
    <script src="./js/y.js"></script>
    <script src="./js/z.js"></script>
</head>
<body>
    <button class id="btn">click</button>

    <div class=" x,a">  
          <span class="a-xxx"></span>

    </div>
    <div class=" x, b">

    </div>
    <div class=" x, c"></div>

    <script>
        function a () {
            console.log('a');
            b();
        }
        function b () {
            console.log('b');
            c();
        }
        function c () {
            console.log('c');
            d();
        }
        function d () {
            console.log('d');
        }

        window.onload = function (e) {
            console.log('大伟聊前端');
            const btn = document.querySelector('#btn');
            btn.addEventListener('click', function () {
                a();
            });
        }







    </script>
</body>
</html>